The long term goal of this project is to understand how an important family of signaling ligands, called Fibroblast Growth Factors (FGFs), control a wide spectrum of cell biological behaviors such as proliferation, cell death, migration, stem cell maintenance and gene expression. In particular we use complex mouse genetics to understand the role of FGF signaling in mesodermal lineages with a special emphasis on extension of the body axis and formation of somites (segmented mesodermal segments that are the building blocks of vertebrate muscle, dermis and vertebral bodies). Our work has made clear that genetic redundancy is an important aspect of this biology; therefore all work in this project emerges from an effort to comprehensively characterize the genetic redundancy of FGF signaling in the mesodermal lineage. Such work is relevant to many cases of cancer where more than one FGF gene may be damaged. To achieve this, we have generated and characterized important Cre mouse lines, which are tools that allow the control of gene expression in the early embryo. These include TCre (expressed in the early emerging nascent mesoderm), TCreERT2 (activatable in emerging nascent mesoderm at all embryonic stages) and Tbx4-Cre (expressed in a posterior mesodermal domain that includes the allantois, hindlimb, and external genitalia. TCre in particular has had a major impact on the field, being essential in over 20 publications. Besides providing the mouse genetics community with valuable mouse lines, this project has yielded papers that document our major insights regarding FGF signaling in the early embryo. We published that Fgf8 not required for somitogenesis, although a body of high profile work had placed it in a central position in current models. However, in collaboration with Alan Perantoni, we demonstrated that Fgf8 was essential for development of the kidney and male reproductive tract. We showed that Fgf8, together with Fgf4, are required for essential aspects of somitogenesis: expression of oscillating gene domains, WNT pathway genes and markers of undifferentiated presomitic mesoderm. Importantly, we demonstrated the premature differentiation of the entire presomitic mesoderm. By examining similar mutants in which we genetically restored WNT signaling, we demonstrated that FGF signaling operates independently of WNT signaling in this process. This functional redundancy that we uncovered has implications for cancer as both FGFs have been found to be aberrantly active in testicular tumors. Furthermore this redundancy has implications for evolution as the same FGFs play compensatory roles in limb development. We are continuing to study genetic redundancy in FGF signaling in several aspects of embryonic development. For example, we are investigating the role of Fgf3 in termination of the embryonic axis. We are investigating the role of Fgf4 and Fgf8 in the differentiation of the somite into its derivative lineages (muscle and bone). In another part of this project, we are studying the role of these Fgfs in development of the allantois, a tissue that gives rise to the placental blood vessels and the umbilical cord. In human, failure of this tissue to develop properly underlies many aspects of pregnancy loss.